Magnetic recording media, such as audio tapes and video tapes loaded in cassettes, have conventionally been used or stored in environments undergoing only mild changes in temperature, such as the interior of a room. However, with the recent spread of radio cassettes, automobile stereos, small-sized video type recorders, and the like, magnetic recording media have been required to withstand use or storage in high temperature or low temperature environments such as the exterior of a room or the interior of automobiles. For example, according to the method of testing automobile parts in high temperatures and low temperatures (JIS D0204), it is reported that the maximum day time temperature in the interior of automobiles can reach as high as 104.degree. C. in summer.
Upon exposure to such a high temperature, the conventional magnetic recording media shrinks, resulting in curling, arching deformation in the longitudinal direction due to unbalanced shrinkage of both edges, and/or squeeze of tapes to cause deformation of hubs or extrusion of hubs. Therefore, if a magnetic recording medium having recorded thereon, for example, music, is exposed to high temperatures, the output level during reproduction widely varies, and not only fails to normally reproduce the music, but also running disorders occur. Particularly in the case of magnetic recording media using a thin support for long-time recording, such as audio compact cassettes, e.g., C-80, C-90, C-120, etc., the length of the magnetic tape loaded in a cassette half increases, and the number of winding times around the hubs also increases so that the above-described problems become noticeable.
Conventionally employed supports for magnetic recording media include stretched and crystallized polyester films, especially those mechanically reinforced in their longitudinal direction by stretching in the longitudinal direction at a high stretching ratio. The inventors have conducted extensive investigations to determine the reasons why the magnetic recording media using these supports are unsatisfactory when used at high temperatures. As a result, it was found that the magnetic recording media using these supports have a percent heat shrinkage of from 1.5 to 2.3% in the direction of maximum shrinkage upon standing at 110.degree. C. for 4 hours.
The inventors further investigated with respect to the relation between surface roughness of a support on the side opposite to a magnetic layer and the running properties and electromagnetic properties, and found that magnetic recording media using a flexible support having a surface that is too smooth cannot provide satisfactory running properties of electromagnetic properties after repeated use.